1. Field of the Invention
The invention relates to the field of electrically interfacing a contactor device with a control apparatus such as a programmable logic controller or "PLC." In particular, the invention concerns a PLC/contactor interface adapted for operation over a range of PLC output voltages, either DC or AC, that provides status information for the PLC, and has a switching means that reduces power dissipation in the interface by a substantial amount.
2. Prior Art
Programmable logic controllers or PLCs are available with various types of input and output stages operable at AC or DC voltages. The outputs typically couple or decouple power to various operating elements. The operating elements can be, for example, contactors or starters for motors, actuators for hydraulic or .pneumatic valves and various other devices that either are powered from the PLC outputs or include switching means that are triggered by the PLC outputs.
The PLC is programmed to activate its outputs in response to conditions at the PLC inputs. For example, the PLC inputs may sense the position of moving elements using limit switches or photocell pairs, sense a signal transmitted from a remote sensor device, or provide a communication path for data.
Where the operating element is a contactor such as a circuit breaker, it is often useful to couple an output signal from the contactor as an input of the PLC, in order to provide an indication of the status of the contactor. The PLC program cannot assume that the contacts are closed simply because an output signal has directed that they be closed, for example because the circuit breaker may be open due to overcurrent or undervoltage line conditions, or due to an interlock otherwise associated with the contactor or circuit breaker that has caused the contacts to be opened.
The circuitry needed to feed back an indication of the status of the contactor can be an expensive addition to the circuit. For example, a relay can be coupled in parallel with the load and in series with the contacts of the contactor to provide a status signal. A contactor also may have a built-in output to indicate its trip status and/or open-closed status. The trip output may be coupled to a bell alarm for generating an alert signal in the event the contactor is tripped. The bell alarm likewise is an additional expense. The open-closed status signal can be an input to the PLC that is tested to enable or disable switching of a further device whose operation may require that the load controlled by the contactor be powered (or perhaps not powered) due to the specific nature of the devices being controlled.
Some other problems with interfacing a contactor and a PLC are due to the AC aspects of operation. Some designers require isolation of the controls from the coil common of the contactor, for example using optical isolators between the PLC and the contactor circuits, to prevent electrical noise from being coupled into the logic circuits of the PLC and to protect the host/operator interfaces in the event of contactor failure. Additionally, many PLCs use solid state outputs to isolate control circuits from noise due to inductive surges that would occur with switching of inductive loads, and/or due to contact bounce. Whereas the PLC output may be at AC line voltage, a triac PLC output stage is often preferred to reduce inductive and contact bounce problems. The triac can be operable for switching the AC line voltage to the load at or near the point of zero voltage crossing.
A triac output, however, must be loaded. A minimum current draw must be maintained after the triac is switched on, in order to keep a triac in conduction. According to one technique, for example, the start/stop signal from a PLC to a contactor is coupled in parallel with a 2.5 K.OMEGA., 7 Watt resistor to ensure that the triac remains loaded sufficiently to remain in conduction. At 120 VAC, the resistor dissipates energy at the rate of 5.76 Watts continuously. This is wasteful of electrical energy and contributes to the need for cooling in control enclosures and the like.
Another difficulty encountered in practical applications of PLC controllers is electromagnetic coupling between signal lines. A PLC may be wired in a plant or the like to control a motor starter or other device located hundreds of feet from the PLC. Signals in both directions between the PLC and the controlled device typically are routed over conductors in close proximity, through wire troughs and/or conduits. Capacitive and inductive coupling between the conductors may be such that a signal on one of the conductors is coupled to another. Optical couplers and solid state switching devices that are preferred to isolate the logic circuits from the line circuits are relatively fast switching devices. Typically the inputs do not require a great deal of input power. In the event that electromagnetic coupling induces a sufficient pulse on the line carrying a Start signal from the PLC to the controlled device, it is possible that the controlled device could be erroneously triggered. Coupling can be reduced by using shielded twisted pair wiring and by ensuring that the parallel runs of signal lines are relatively short. However, this can be expensive. Slower operating switching devices and higher input thresholds also are possible, but have obvious drawbacks with respect to the power requirements and operational characteristics of the system.
It would be advantageous to provide an interface circuit that satisfies the signalling and status reporting needs of a PLC interface as described, particularly for a starter or circuit breaker device that can be tripped, that keeps a PLC triac output true once it is switched on, that reduces the dissipation of electrical energy and heat, and that is insensitive to noise.